How big are the human eyes supposed to be?

I am trying to draw an attractive human face as accurately as possible, and trying to get all the features/proportions correct. However, I keep getting stuck at the eyes. How wide/"tall" are the eyes supposed to be? I know the Phi ratio, and how the middle of the eyes are supposed to be 1.618 x the middle of the lips from the chin. Does anyone know how "tall" the eyes are supposed to be? I can probably figure out width since the 1/5 thing across the face. Thanks!